1. Field of the Invention
This invention relates to optical endoscopy.
2. Discussion of the Related Art
Multi-photon imaging exploits non-linear optical properties of a sample to create an image of the sample. One type of multi-photon imaging is two-photon fluorescent microscopy in which scanning light causes portions of a sample to fluoresce as a result of two-photon absorption events in the sample. Other types of multi-photon imaging use other multi-photon processes, e.g., three-photon fluorescence, second- or third-harmonic generation, and Raman absorption, to create images. These multi-photon processes enable producing scanned images of samples.
Imaging techniques based on nonlinear optical properties have several common features. One common feature is that the produced images depend on the chemical composition of a sample. Thus, the images enable extracting data on a sample's chemical composition, i.e., data that may not be available through imaging techniques based on linear optical processes. Another common feature is the use of lower energy photons than in imaging techniques based on linear optical processes. Lower energy photons are used, because more than one photon provides the excitation energy for the nonlinear optical processes. The lower energy photons have longer wavelengths that typically penetrate better in dense sample media such as biological tissue. Another common feature is that the optical imaging events have smaller optical cross-sections than those used in imaging techniques based on linear optical processes. The smaller optical cross-sections usually necessitate higher illumination intensities than in the imaging techniques based on linear optical processes. For the higher illumination intensities, non-linear optical imaging systems typically rely on ultra-fast pulsed lasers, e.g., pulsed femto-second or pulsed pico-second lasers.
Pulses from such ultra-fast pulsed laser sources are susceptible to degradation by dispersion and non-linear optical processes that occur in imaging instrumentation. Dispersion and non-linear optical processes produce temporal and spectral alterations of optical pulses. These degradative effects reduce the ability of the pulses to generate multi-photon events in a sample. Although dispersion can be pre-compensated, non-linear optical processes are usually not amenable to pre-compensation. For that reason, the non-linear processes interfere with multi-photon imaging techniques and have impeded the use of optical endoscopes in multi-photon imaging.